In some wireless communication systems, one or more communication devices employ multiple antennas. Accordingly, a communication channel between two such devices can be i) a multiple-input, multiple-output (MIMO) channel when both communication devices employ multiple antennas, ii) a single-input, multiple-output (SIMO) channel when a transmitting device (“the transmitter”) employs a single transmit antenna and the receiving device (“the receiver”) employs multiple receive antennas, or iii) a multiple-input, single-output (MISO) channel the transmitter employs multiple transmit antennas and the receiver employs a single receive antenna. Referring for simplicity to transmit beamforming, transmission and reception properties in these systems can be improved by using each of the various transmit antennas to transmit the same signal while phasing (and amplifying) this signal as the signal is provided to the various transmit antennas to achieve beamforming or beamsteering. Generally speaking, beamforming or beamsteering creates a spatial gain pattern having one or more lobes or beams (as compared to the gain obtained by an omni-directional antenna) in one or more particular directions, while generally reducing the gain over that obtained by an omni-directional antenna in other directions. If the gain pattern is configured to produce a high gain lobe in the direction of each of the receiver antennas or in the direction of the receiver antennas in general, the MIMO system can obtain better transmission reliability between a particular transmitter and a particular receiver, over that obtained by single transmit-antenna/receive-antenna systems.
To conduct beamforming in the direction of a receiver, a transmitter generally utilizes a steering matrix determined based on specifics of the forward channel (i.e., the channel from the transmitter to the receiver) to condition the signals applied to various transmit antennas so as to produce the desired transmit gain pattern. In a technique known as explicit beamforming, to determine the specifics of the forward channel, such as the channel state information (CSI) or other measured description of the forward channel, the transmitter first sends training data to the receiver, which receiver then determines or estimates characteristics of the forward channel and/or determines a steering matrix that specifies beamsteering coefficients to be used by the transmitter, and then transmits this information back to the transmitter. The transmitter, in turn, receives the forward channel information (or steering matrix) from the receiver and utilizes this information to create the desired gain pattern in subsequent transmissions to the receiver. Explicit beamforming typically uses one of three types of feedback channel descriptions, which are describe in the context of an access point (AP) and a client station for ease of explanation. With channel state information (CSI) feedback, the client station estimates the downlink (e.g., forward) channel from a sounding packet transmitted by the AP and feeds back the estimated channel gains. With uncompressed steering matrix feedback, the client station, based on the channel estimate from a sounding packet from the AP, determines the steering matrix that is to be used at the AP. The client station then feeds back this steering matrix, without compression. With compressed steering matrix feedback, a similar process occurs, but the steering matrix is fed back in a compressed form.
On the other hand, in implicit beamforming, the transmitter determines specifics of the reverse channel (the channel from the receiver to the transmitter) based on training signals that the transmitter receives from the receiver and estimates the forward channel from the reverse channel by assuming channel reciprocity.
Beamforming typically involves a protocol for transmitting training signals and feeding back CSI or beamsteering coefficients (i.e., a beamforming training protocol). Some communication devices, however, do not support beamforming training protocols and therefore cannot explicitly participate in beamforming training. Some beamforming techniques are described in “IEEE Std. 802.11n™ IEEE Standard for Information Technology-Telecommunications and information exchange between systems-Local and metropolitan area networks-Specific requirements, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: Amendment 5: Enhancements for Higher Throughput,” The Institute of Electrical and Electronics Engineers, Inc., October 2009.